[SPARK-13413] Remove SparkContext.metricsSystem

## What changes were proposed in this pull request?

This patch removes SparkContext.metricsSystem. SparkContext.metricsSystem returns MetricsSystem, which is a private class. I think it was added by accident.

In addition, I also removed an unused private[spark] method schedulerBackend setter.

## How was the this patch tested?

N/A.

Author: Reynold Xin <rxin@databricks.com>

This patch had conflicts when merged, resolved by
Committer: Josh Rosen <joshrosen@databricks.com>

Closes #11282 from rxin/SPARK-13413.